1. Field of the Invention:
The present invention relates generally to nuclear reactor fuel assemblies, and more particularly to a new and improved end plug which is adapted to be welded within the ends of nuclear reactor fuel rods in order to close the same when fabricating the fuel rods for subsequent incorporation within the nuclear reactor core fuel assemblies, wherein the particular end plug of the present invention facilitates the elimination of structural defects and adverse properties conventionally characteristic of the fuel rod-end plug assembly circumferential or girth weld beads, especially when the welding operations are performed in accordance with laser welding techniques, although the teachings of the present invention may also be applicable to those fuel rod-end plug assembly weld beads when the welding operations are performed in accordance with TIG welding techniques.
2. description of the Prior Art:
As is well known in the nuclear power plant art, the nuclear reactor core of a typical nuclear reactor facility or power plant may comprise, for example, at least one hundred or more fuel assemblies, and each fuel assembly, in turn, may comprise, for example, several hundred fuel rods. A conventional fuel assembly may comprise, for example, what is known as a 17.times.17 array of fuel rods wherein there are seventeen rows of fuel rod locations, and each row contains seventeen fuel rod locations. Consequently, each fuel assembly comprises two-hundred sixty-four fuel rods, with the remaining locations housing control rod thimble tubes and an instrumentation tube. Thus, if the reactor core comprised at least one hundred fuel assemblies, it can readily be seen that the core would comprise more than twenty-five thousand fuel rods.
The nuclear reactor core fuel pins or fuel rods conventionally comprise thin-walled tubing or cladding within which the fissionable material is housed and supported in a totally encased manner during operation of the reactor. The cladding serves to prevent contact and chemical reactions from occurring between the nuclear fuel and the surrounding environment, such as, for example, the coolant water in a pressurized water reactor (PWR), and of course the cladding additionally serves to confine the radioactive fissionable material therewithin. In order to achieve the aforenoted operational objectives, the cladding must therefore be corrosion-resistant, non-reactive, and heat conductive. Conventionally employed materials utilized in the fabrication of the fuel rod cladding include, for example, type 304 stainless steel, or zirconium based alloys, such as, for example, zircaloy-2 or zircaloy-4. The zirconium based alloys are in fact preferable over the stainless steel materials in view of the fact that the zirconium alloys exhibit relatively low capture cross-sections with respect to thermal neutrons.
In accordance with actual conventional fuel pin or fuel rod fabrication techniques, the nuclear fuel, which is usually in the form of cylindrical pellets or uranium dioxide (UO.sub.2) enriched with U-235, is hermetically sealed within the fuel rod cladding or tubing by inserting the fuel within the cladding or tubing and subsequently capping both ends of the cladding or tubing with end plugs or closures which are temporarily attached to the tubing or cladding by means of a friction or force fit. The end plugs are then permanently sealed in position within the tubing or cladding by means of welding operations which are performed, for example, by means of automatic welding apparatus which rotates the fuel rod tubing relative to an electrode so as to thereby form a girth weld at the cladding-plug interfaces, the resulting assemblage of the cladding and end plugs thereby defining the completed fuel rods or pins.
To date, and prior to the invention embodied within the present patent application, all of the aforenoted thousands of fuel tubing-end plug girth weld interfaces have in fact been achieved in accordance with conventional tungsten inert gas (TIG) welding techniques. TIG welding techniques have of course been in existence for many years and have been employed in connection with welding operations within many various fields or environments. TIG welding operations are in fact highly reliable and are relatively trouble-free. Production cycle time is relatively fast, however, with the advent of laser welding technology, it has been readily realized that TIG welding operations are not nearly as fast as laser welding operations. Consequently, in order to render welding operations more cost effective and economical, it has been desired to employ laser welding techniques wherever possible in lieu of TIG welding techniques. It can be further appreciated that in connection with the particular fabrication processing of nuclear reactor fuel rods, particularly in view of the number of weld interfaces that are required in connection with the number of fuel rods comprising a single nuclear reactor power plant core, considerable economic advantages are capable of being achieved if in fact laser welding techniques can be employed. Unfortunately, prior to the invention embodied within the present application, the fuel rod-end plug girth welds exhibited serious structural defects or deficiencies when laser welding techniques were in fact employed in lieu of conventionally acceptable TIG welding techniques heretofore employed in achieving such weld interfaces.
The foregoing may be better appreciated if reference is made, for example, to FIG. 1 of the drawings which discloses a conventional long-land end plug normally employed for sealing the ends of fuel rod tubes or cladding wherein the assembled fuel rods will be subsequently employed within a 17.times.17 core fuel assembly. The conventional end plug is generally indicated by the reference character 10 and is seen to be in the form of a substantially right circular cylinder having a larger diameter portion 12 and a small diameter portion 14. Small diameter portion 14 is provided with a tapered forward end 16 wherein the angle of taper .alpha. may be, for example, 10.degree., the tapering of the forward end of the plug 10 serving to facilitate insertion of the plug 10 within the fuel rod cladding or tubing, not shown. The rearwardly extending remainder portion 18 of small diameter portion 14 of the plug 10 forms a circumferential land area which will frictionally engage the interior surface of the cladding or tubing end when the end plug 10 is inserted within the cladding or tubing, not shown. Due to the different diametrical extents of end plug portions 12 and 14, a circumferentially extending flange or shoulder portion 20 is defined therebetween, and the end plug 10 is further provided with a bore 22 which serves to introduce high-pressure inert gas into the fuel rod cladding or tubing for completion of the assembled cladding-end plug fuel rod. An enlarged co-axial bore 24 serves to provide fluidic communication between the gas inlet orifice or bore 22 and the interior of the fuel rod, it of course being understood that subsequent to the charging of the fuel rod interior with the pressurized inert gas, bore or orifice port 22 is sealed by means of a suitable welding operation.
In the fabrication of fuel rod end plugs 10, it has been imperative that quality control be high, particularly in connection with the tolerances for the production of the flanged or shoulder portion 20 as well as the land area 18 so that, in turn, the corner defined between such relatively perpendicularly oriented surfaces is square and sharp. Similarly, the edge surfaces of the cladding or tubing which are to mate with the corresponding surfaces and corner structure of the end plugs should likewise be accurately defined so as to eliminate unacceptable tolerance gaps defined between such mating surfaces. These manufacturing tolerances have in fact proven to be critical in view of the fact that should such tolerances not be achieved or adhered to, various internal structural defects will result in light of the TIG welding operations being performed. For example, porosity conditions will be observed within the weld bead, which can lead to structural integrity and strength problems, and similarly, a condition known as ID undercut, wherein there is exhibited a reduction in the thickness of the cladding or tubing portion adjacent the weld area, can likewise manifest itself so as to also lead to structural integrity and strength problems, it being particularly remembered that the enclosed fuel rods are internally pressurized wherein structural weaknesses within the fuel rod casings and welds would be especially undesirable. As a result of such adherence to high quality control standards, such structural defects have in fact been minimized in connection with the fabrication of conventional fuel rods utilizing TIG welding techniques.
In seeking to employ laser beam welding technology and techniques to the aforenoted type of fuel rod cladding-end plug assemblies, it has been observed that while ID undercut defects or deficiencies have not manifested themselves within the weld metal, porosity defects or deficiencies have in fact been enhanced relative to similar welding techniques employing TIG welding. Such porosity defects can be seen with reference being made to FIGS. 2 and 3 which disclose spheroidized gas pores or, in other words, a string of porosity defects, the photographs disclosing a mid-plane surface of the laser weld under 10x 50x magnification conditions, respectively, appearing in the weld bead when laser beam welding techniques were employed in connection with conventional fuel rod end plugs as disclosed in FIG. 1 being mated with conventional fuel rod cladding or tubing. While no explicitly definite reasons have been tendered in connection with the occurrence of such phenomena adversely affecting the structural properties of the weld beads, gas present within the vicinity or region of the inner corner of the end plug as defined by the shoulder portion 20 and the land portion 18 thereof apparently becomes trapped within the molten metal pool due to, for example, the various vaporization, solidification, and shrinkage properties characteristic of the fuel rod end plug and cladding tubing components, as well as the weldment defined there-between, when subjected to laser beam welding operations. Consequently, if in fact laser beam welding techniques are to be employed in connection with the fabrication of nuclear reactor fuel rod-end plug assemblies, the occurrence of such aforenoted porosity defects within the weld bead must be prevented.
Accordingly, it is an object of the present invention to provide a new and improved nuclear reactor fuel rod end plug.
Another object of the present invention is to provide a new and improved nuclear reactor fuel rod end plug which will eliminate the various structural deficiencies of conventional fuel rod-end plug assemblies when the assemblies are welded together by means of laser beam welding techniques.
Yet another object of the present invention is to provide a new and improved nuclear reactor fuel rod end plug which will in fact facilitate the employment of laser beam welding techniques in connection with the fabrication of nuclear reactor fuel rod-end plug assemblies without developing any substantial adverse or deleterious structural deficiencies within the assemblies.
Still another object of the present invention is to provide a new and improved nuclear reactor fuel rod end plug which will enable laser beam welding techniques to be employed in connection with the fabrication of nuclear reactor fuel rod-end plug assemblies so that such assemblies may in fact be fabricated in a substantially cost-effective manner.
Yet still another object of the present invention is to provide a new and improved nuclear reactor fuel rod end plug which will enable laser beam welding techniques to be employed in connnection with the fabrication of nuclear reactor fuel rod-end plug assemblies regardless of whether or not the nuclear reactor fuel rod cladding or tubing, as well as the end plugs, are fabricated from stainless steel or zirconium alloys.
Still yet another of the present invention is to provide a new and improved nuclear reactor fuel rod end plug which will enable TIG welding techniques to be employed in connection with the fabrication of nuclear reactor fuel rod-end plug assemblies in a substantially improved cost-effective manner as compared to conventional fabrication processing of nuclear reactor fuel rod-end plug assemblies by TIG welding in view of the fact that the machining costs are able to be substantially reduced because the end plug corners and surfaces, as well as the surfaces and edges of the fuel rod cladding or tubing, are no longer required to have the requisite critical dimensions, within predetermined critical tolerances, as was characteristic of conventional nuclear reactor fuel rod cladding or tubing and end plugs when such fuel rod-end plug assemblies were to be assembled employing TIG welding techniques.
A further object of the present invention is to provide a new and improved nuclear reactor fuel rod end plug which imparts to the nuclear reactor fuel rod-end plug assemblies improved joint characteristics.
A yet further object of the present invention is to provide a new and improved nuclear reactor fuel rod end plug which facilitates the nuclear reactor fuel rod cladding or tubing-end plug assemblies to be welded together by means of laser beam welding apparatus which may operate at considerably reduced power levels.
A still further object of the present invention is to provide a new and improved nuclear reactor fuel rod end plug which will facilitate the achievement of full penetration welds within the nuclear reactor fuel rod cladding or tubing-end plug joint regions at reduced power levels, and which will further facilitate the more precise verification of such full penetration weld achievement by means of conventional radiographic techniques.